Nuclear power plants are periodically shut down for refueling, maintenance and intensive inspection and testing, and wherever possible these functions are to be carried out simultaneously rather than sequentially to minimize outage time. Since the reactor pressure vessel and refueling pool are flooded with water during refueling, some form of temporary seal must be installed in the piping connecting the reactor vessel with the generator in order to isolate that pool from the steam generator and permit concurrent testing and inspection of the generator components. This seal is achieved by installing what is known in the industry as a nozzle dam in the nozzles of the steam generator primary head. Its design should be such that its parts can be carried through a small manway in the generator head and assembled by hand very quickly to minimize personnel radiation exposure. It also must effect a reliable water-tight seal without compromising the structural integrity of the nozzle wall or steam generator wall.
One form of nozzle dam is described in a paper presented at the American Power Conference in Chicago, Ill. in April of 1983 by John W. Alden and Richard P. Kosky. It requires that a ring of radial holes be drilled in the nozzle wall just inside the opening into the generator. The dam itself is assembled in segments (two or three) inside the generator and includes a membrane with an inflatable peripheral seal. It is releasably secured in place by radial locking pins which enter inserts in the drilled holes in the nozzle wall. A major shortcoming of this form of nozzle dam is the use of drilled holes which have the potential at least of weakening the nozzle wall. Stress analyses must be performed after the holes are drilled to insure that the nozzle wall structural integrity has not been compromised. Also the pins are under a shear load from the hydrostatic water pressure.
Another form of nozzle dam is described in the October 1983 issue of the journal Radiation Protection Management, in an article at pages 65 to 76 entitled "Radiological Experiences with Steam Generator Nozzle Dam Installations at Salem Nuclear Generator Station" by W. L. Britz and H. W. Bergendahl. This design avoids holes in the nozzle wall by using a permanently welded-in-place flange ring affixed to the inside of the generator bowl around the opening of the nozzle. A rubber diaphragm is placed over the flange ring and is overlaid by a 70-pound folding circular cover bolted onto the ring through the edge of the diaphragm by twenty bolts. Among the disadvantages of this design is the use of threaded bolt holes which can be subject to galling. Since the holes in the ring are permanently in the generator they may require cleaning before use. This adds a step to the dam installation procedure and thus increases personnel radiation exposure time. It is also quite time-consuming to screw in the circle of bolts. Another reported problem of this design is that the flange ring is not always sufficiently flat to permit a uniform sealing pressure on the diaphragm.
It is the purpose of the present invention to provide a new design of nozzle dam which avoids the drilling of holes in the nozzle wall or generator wall, which involves no threaded bolt holes permanently in the generator, and which permits a uniform pressure seal regardless of irregularities in flatness of a flange ring around the nozzle opening.